1. Field of the Invention
The present invention relates to a glass composition. More particularly, the present invention relates to a glass composition for use as a cane glass seal of the type used, for example, in fusing together the plates of gas panel display devices and the like.
2. Description of the Prior Art
The fabrication of gas panel display devices requires that two glass plates be fused together and sealed. The glass used to fuse the two plates together is known as solder glass. Solder glass should have a low softening temperature so that fusing occurs at a temperature which will not alter the glass plates or other components to be assembled.
It is also important that the thermal expansion characteristics of the solder glass approxiate that of the plate glass or other glass to be fused. Otherwise, differential expansion between the solder glass and plate may cause excessive stresses which will fracture the seal or plates. This match of thermal characteristics should exist over a temperature range from room temperature to the temperature where the solder glass is rigid enough for strain to form.
Essentially, there are two separate techniques for forming glass seals of the type described. One involves the use of glass in a frit form which is applied (e.g. by screening) to the plate glass. The plates to be sealed are then stacked together and heated under appropriate conditions to form a permanent bond.
The second approach, and the one to which this invention pertains, involves the use of a so-called cane seal. In this case, a "cane" of a suitable solder glass is drawn, formed to the required shape, and placed between the two glass plates to be sealed. The assembly is then raised to the appropriate fusing temperature for the proper time to form an hermetic seal.
The cane seal has advantages over the frit seal in that the porosity associated with the frit seal is avoided and a better seal is produced. Furthermore, the glass frit must be applied in an organic binder which must be subsequently baked out. The cane seal requires no organic binder and thus eliminates a source of contamination as well as the added bake-out cycle.
Many conventional composition manipulations that would reduce the softening point or transition temperature of a glass would also tend to increase the coefficient of expansion of the solder glass beyond tolerable limits. Thus, conventional solder glasses have not proved acceptable for fusing together the glass plates of display panels and the like. One reported approach has involved the doping of a conventional alkali free lead borate solder glass with copper oxide. Through careful control of the valence state of the copper by oxidation--reduction equilibria, the softening point of the glass can be lowered while a close match is maintained between the expansion characteristics of the seal and plate glass.
A disadvantage of this process for manufacturing cane glass seal is that it is difficult to control the valence state of copper oxide.
Moreover, the various sealing glass compositions of this general type (with or without copper oxide) have also included silica (about 2-6% by weight) as a glass former, possibly to serve as a stabilizing agent. One aspect of this invention is based on the discovery that the presence of as little as 2% silica may tend to result in crystallization problems in the seal.
Examples of sealing glass compositions of the type described may be found in U.S. Pat. Nos. 3,873,330, 3,904,426 and 3,778,242. See also "Role of Copper Ions in Low-Melting Solder Glasses" by Takamori et al, Journal of The American Ceramic Society, Vol. 59, No. 7-8, pp. 312-316.